The present invention relates to a process for the production of tubular structural parts fabricated from PGM materials and having circumferential undulating bulges by forming from smooth-walled tube pieces.
Structural parts fabricated from precious metal materials, such as preferably PGM materials, are used in the glass industry, in particular in plants for the fusion and hot forming of special glasses.
On account of their high melting point, materials of PGM metals (platinum group metals) are characterised by a high thermal resistance and also by high mechanical strength and resistance to abrasion, and are therefore particularly suitable for the production of structural parts in plants or plant units that come into contact with glass melts. Suitable materials are platinum and alloys of platinum and/or other PGM metals, which may optionally also contain minor amounts of non-precious metals as further alloying components or oxide additives. Typical materials are refined platinum, PtRh10 (platinum-rhodium alloy with 10% rhodium) or platinum, which contains a small amount of finely divided refractory metal oxide, such as in particular zirconium oxide (so-called fine grain-stabilized platinum), in order to improve the mechanical strength and high-temperature creep resistance.
Such melt technology plant components serve for the fusion, refining, transportation, homogenization and charging of the molten glass.
Such structural parts are substantially precious metal sheet-type constructions that are often fabricated as thin-walled tubular systems. The molten glass flows through such systems at temperatures of between 1000° C. and 1700° C. These tubular systems are as a rule surrounded by an insulating as well as supporting ceramic material, which in turn is frequently held by supporting metal structures such as metal boxes.
The PGM structural parts are fabricated at room temperature and installed in the corresponding units. However, the units are operated at temperatures in the range from about 1000° to 1700° C.
Thin-walled sheet metal structures have only a low dimensional rigidity, in particular at high operating temperatures. In order to compensate for this disadvantage the material thickness must either be increased or the structure must be stabilized by stiffening forming measures such as for example the formation of bends, edges, corrugations or folds.
Furthermore, when designing and building corresponding units the high thermal expansion of the PGM structural parts as well as the different thermal expansion of all the other materials involved (precious metals, ceramics, steels, etc.) must be taken into account. The mean coefficient of thermal expansion of platinum at a temperature of 1500° C. is 11.2×10−6 K−1. This means that a platinum structural part that is one meter long at room temperature has expanded by 16.6 millimeters at 1500° C.
Due to the different coefficients of thermal expansion of the various materials and structural securement points present on a structural part, a free expansion of the system is not possible. Accordingly bending or even buckling may occur at weak points in PGM sheet structures, and this in turn leads to the undesired premature failure of the system. In plants or parts of plants fabricated from PGM materials that come into contact with the glass melt, structural parts therefore have to be provided that compensate for the linear expansion.
Tubular sections that have circumferential undulating bulges, such as for example corrugated tubes or bellows, may be used as structural elements in tubular plant parts to impart a radial stiffening and to a certain extent also to compensate for linear expansion.
The forming of corresponding smooth-walled tube pieces into corrugated tubes is carried out according to the prior art by so-called roll crimping or roll forming. In this, the wall of the smooth-walled tube piece is forced out by a curling tool acting from the inside, into the radial corrugated recess of a forming die. In roll crimping each individual corrugation is rolled successively step by step.
A tube formed in this way and thus stiffened in the radial direction becomes more elastic in the axial direction and can therefore also be used for length compensation.
Roll crimping has however—specifically with regard to the production of corrugated structural parts from PGM materials for use in melt technology plants in the glass industry—a number of disadvantages and limits on potential use.
Thus, only relatively small shape alterations, for example in the region of sinusoidal wave contours, can be effected by roll crimping. Higher corrugations peaks, sharper folds or even arbitrary contour shapes cannot be produced in practice. For this reason corrugated tubes produced by roll crimping are of only limited suitability for compensating thermal linear expansion since the corresponding corrugation geometries can compensate only for moderate linear expansions.
Furthermore, roll crimping is not possible with small tube diameters.
Due to the stretching of the material in roll crimping there is inevitably a thinning (reduction in wall thickness) in the region of the corrugations. The structural part is thus considerably weakened, which can lead to a premature failure under the thermal and abrasive stresses produced by contact with the glass melt.
An object of the invention is accordingly to provide structural parts of PGM materials for use as linear expansion compensators in units or parts of units coming into contact with the glass melt, and also to provide a production process for such structural parts in which the aforedescribed disadvantages are avoided.